1. Technical Field
This disclosure relates to an alumina sintered body containing a main phase consisting of alumina and crystalline phases dispersed in the main phase.
2. Related Art
Alumina sintered bodies have excellent heat-resistance and dielectric strength, and accordingly are used in insulators such as for vehicle parts. Recently, spark plugs for vehicles need downsizing, and it is desired that thickness of the insulators is smaller. Also, the spark plugs is desired to use a higher voltage. Accordingly, the alumina sintered bodies for insulators of the spark plugs need higher dielectric strength.
Conventionally, an insulating material containing alumina as a primary constituent has been suggested, with a first additive constituent consisting of Si, Ca, Mg, Ba, B and the like, and a second additive constituent consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn or the like (Japanese Patent Application Publication No. 2000-247729). In the insulating material, by adding both Mn constituent and Cr constituent, a composite oxide phase having high melting point is formed. This increases dielectric strength of the insulating material.
However, in the conventional insulating material, the crystalline phases are formed at random in the alumina sintered body, and accordingly variation of dielectric strength is likely to occur in the alumina sintered body. As a result, the whole sintered body might be insufficient in dielectric strength.